REGULATORY CAPITAL (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Actual and required capital amounts and ratios
Number of ratios calculated according to the regulations for determining assigned capital category | item	4
Total capital (to risk-weighted assets)
Actual, Amount	$ 360,610	$ 356,210
Required for Adequate Capital, Amount	$ 185,486	$ 184,170
Actual, Ratio	15.60%	15.50%
Required for Adequate Capital, Ratio	8.00%	8.00%
Tier 1 capital (to risk-weighted assets)
Actual, Amount	$ 339,531	$ 334,190
Required for Adequate Capital, Amount	$ 139,115	$ 138,127
Actual, Ratio	14.60%	14.50%
Required for Adequate Capital, Ratio	6.00%	6.00%
Common equity Tier 1 capital (to risk-weighted assets)
Actual, Amount	$ 298,657	$ 293,316
Required for Adequate Capital, Amount	$ 104,336	$ 103,595
Actual, Ratio	12.90%	12.70%
Required for Adequate Capital, Ratio	4.50%	4.50%
Tier 1 capital (to average assets)
Actual, Amount	$ 339,531	$ 334,190
Required for Adequate Capital, Amount	$ 132,833	$ 130,845
Actual, Ratio (as a percent)	10.20%	10.20%
Required for Adequate Capital, Ratio	4.00%	4.00%
Cash and cash equivalents	$ 72,002	$ 67,578	$ 69,550	$ 62,485
Minimum | Prior To Federal Reserve Corrective Action Rules
Tier 1 capital (to risk-weighted assets)
Required for Adequate Capital, Ratio	4.00%
Minimum | Federal Reserve Corrective Action Rules
Basel III and Dodd Frank Rules
Required minimum Tier 1 capital to risk weighted assets (as a percent )	4.50%
Required minimum conservation buffer percentage	2.50%
Tier 1 capital (to risk-weighted assets)
Required for Adequate Capital, Ratio	6.00%
Tier 1 capital (to average assets)
Required for Adequate Capital, Ratio	4.00%
Parent Company
Tier 1 capital (to average assets)
Cash and cash equivalents	$ 19,800
MainSource Bank
Basel III and Dodd Frank Rules
Tier 1 leverage capital required to be well capitalized to average assets (as a percent)	5.00%	5.00%
Total capital (to risk-weighted assets)
Actual, Amount	$ 332,608	$ 334,266
Required for Adequate Capital, Amount	185,485	184,168
To Be Well Capitalized, Amount	$ 231,856	$ 230,211
Actual, Ratio	14.30%	14.50%
Required for Adequate Capital, Ratio	8.00%	8.00%
To Be Well Capitalized, Ratio	10.00%	10.00%
Tier 1 capital (to risk-weighted assets)
Actual, Amount	$ 311,529	$ 312,246
Required for Adequate Capital, Amount	139,114	138,126
To Be Well Capitalized, Amount	$ 185,485	$ 184,168
Actual, Ratio	13.40%	13.60%
Required for Adequate Capital, Ratio	6.00%	6.00%
To Be Well Capitalized, Ratio	8.00%	8.00%
Common equity Tier 1 capital (to risk-weighted assets)
Actual, Amount	$ 311,529	$ 312,246
Required for Adequate Capital, Amount	104,335	103,595
To Be Well Capitalized, Amount	$ 150,707	$ 149,637
Actual, Ratio	13.40%	13.60%
Required for Adequate Capital, Ratio	4.50%	4.50%
To Be Well Capitalized, Ratio	6.50%	6.50%
Tier 1 capital (to average assets)
Actual, Amount	$ 311,529	$ 312,246
Required for Adequate Capital, Amount	132,542	130,555
To Be Well Capitalized, Amount	$ 165,678	$ 163,194
Actual, Ratio (as a percent)	9.40%	9.60%
Required for Adequate Capital, Ratio	4.00%	4.00%
To Be Well Capitalized, Ratio	5.00%	5.00%